The wavelength of light and the repeater intervals for repeating and transmitting light in optical fiber communications are dependent on the transmission loss and the band characteristic of the optical fiber used. Especially, the dispersion characteristic of the optical fiber causes a waveform distortion, thereby imposing a limitation on the transmission rate in digital transmission. Accordingly, even if an optical fiber of an extremely small loss is employed, the repeater intervals may sometimes be limited by the dispersion characteristic of the optical fiber; therefore, measurement of the dispersion characteristic of the optical fiber is very important as is the case with measurement of the transmission loss. These measurements are usually conducted, using a small-length fiber, and the measured values are applied to a large-length fiber on the basis of the ratio of length, so that even a small measurement error in the small-length fiber will produce a large error in determining the dispersion characteristic of the large-length fiber. For the reason mentioned above, a measuring apparatus with a sufficiently high accuracy is required for measuring the dispersion characteristic of the small-length fiber.
However, conventional measuring apparatus cannot be employed as described below for measurements in wavelength of 1.3 .mu.m and 1.5 .mu.m bands which are regarded as promising for optical fiber communications.